app: Now disallows duplicate posts

This makes the proof-of-work system actually useful :3

app.py

@@ -39,14 +39,25 @@ def get_all_quotes() -> list[Quote]:
     return quotes
 
 
-def add_quote(new_quote: Quote):
-    db_connexion = get_database_connection()
-    db_connexion.execute(
+def add_quote(new_quote: Quote) -> bool:
+    db_connection = get_database_connection()
+
+    matching_quote = db_connection.execute(
+        "SELECT author, content, timestamp FROM quotes WHERE author = ? AND content = ?",
+        (new_quote.author, new_quote.content)
+    ).fetchall()
+
+    if matching_quote:
+        # There is already an author and a quote with the same content, let's not do duplicates...
+        return False
+
+    db_connection.execute(
         "INSERT INTO quotes (author, content) VALUES (?, ?)",
         (new_quote.author, new_quote.content)
     )
-    db_connexion.commit()
-    db_connexion.close()
+    db_connection.commit()
+    db_connection.close()
+    return True
 
 
 @app.route('/', methods=["POST", "GET"])
@@ -62,8 +73,11 @@ def index():
         else:
             quote = Quote(form["username"], form["message"])
             if quote.proof_of_work_is_valid(form["proof-of-work"]):
-                add_quote(Quote(form["username"], form["message"]))
+                if add_quote(Quote(form["username"], form["message"])):
                     message = "Your quote was added successfully"
+                else:
+                    message = "Couldn't add your quote. Is it a duplicate?"
+                    response_code = 400
             else:
                 message = "You didn't do the work, silly!"
                 response_code = 400